Type-1 diabetes (T1D) is a chronic autoimmune disorder thought to be caused by autoreactive T-cells that participate in destruction of insulin-producing pancreatic β-cells in the islets of Langerhans. Complete destruction of β-cells results in a lifelong dependence on exogenous insulin. Specifically, CD4+ T-cells are known in the art to play a critical role in T1D pathology (Anderson, et al., 2005, Annual review of immunology 23: 447-85). Prior to diagnosis of T1D, pancreatic islets are infiltrated by inflammatory cells including CD4+ T-cells and antibodies to various β-cell antigens. These proteins are present in the sera of patients at risk (Kent et al., 2005, Nature 435: 224-8; Achenbach et al., 2005, Current diabetes reports 5:98-103).
It has been suggested that autoimmune destruction of β-cells and diabetes onset may be associated with a reduction in regulatory T-cell (Treg) numbers and/or functions (Waid et al., 2008, Journal of leukocyte biology 84:431-9; Brode et al., 2006, Journal of immunology 177:6603-12). On the other hand, antigen presenting cells (APCs) in NOD mice and T1D patients have been shown to be defective in their ability to stimulate CD4+CD25+ Treg function although the Tregs themselves appear to be functional (Alard et al., 2006, Diabetes 55:2098-105; Manirarora et al., 2008, PloS one 3: e3739). The inventors and others in the art have shown that restoring Treg function or inducing adaptive Tregs may be an effective method for preventing and or stabilizing autoimmune diabetes (Cheatem et al., 2009, Clin Immunol 131: 260-70; Gaudreau et al., 2007, J. Immunol. 179: 3638-47; Karumuthil-Melethil et al., 2010, Journal of immunology 184:6695-708; You et al., 2007, Proceedings of the National Academy of Sciences of the United States of America 104:6335-40).
Existing and emerging therapies in the art utilize broad-based immunoregulatory strategies, such as inhibition or deletion of lymphocytes subsets and/or establishing immune tolerance via activation of Tregs and include for example non-mitogenic anti-CD3 or anti-thymocyte globulin (Chatenoud 2003, Nature reviews. Immunology 3: 123-32; Chung et al., 2007, International immunology 19: 1003-10). These approaches, however, result in global attenuation of the immune response and render the patient susceptible to opportunistic infections and cancers. In this regard, biologics including monoclonal antibodies (mAbs) and tumor necrosis factor (TNF) inhibitors are commonly prescribed to individuals suffering from autoimmune diseases (O'Shea et al., 2002, Nature reviews. Immunology 2:37-45). Three licensed mAbs (adalimumab, etanercept and infliximab) are currently on the market for the treatment of immune mediated inflammatory diseases, including diabetes mellitus (Silva et al., 2010, Immunotherapy 2: 817-33). However, in September 2008, the FDA announced that manufacturers of TNF inhibitors must strengthen existing warnings on the risk of fungal infections, in particular, histoplasmosis.
As a result, safer, targeted therapeutics are needed in the art. A highly desired alternative approach is the induction of T-cell tolerance to β-cell antigens for prevention of disease development in patients at risk or with recent onset of disease. Central to the immune response are T-cells whose activity can be up- or down-regulated resulting in an immune response that attacks or ignores an antigen, respectively. The ability to down-regulate or become tolerant to specific antigens is crucial for preventing or treating autoimmune diseases. Current clinical approaches for these indications are not target specific and result in global attenuation of the immune response, however. It is therefore beneficial to selectively suppress self-reactive T-cells while leaving the rest of the immune system intact. To do so, T-cell expressed cell surface markers provide a convenient and specific target for inducing desired immunomodulation.
Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) is a cell surface marker molecule expressed on activated T-cells that plays a critical role in maintaining peripheral tolerance and has been linked to insulin-dependent diabetes (IDD) disease susceptibility in both human patients and NOD mice (Todd et al., 2001, Immunity 15: 387-95; Karumuthil-Melethil et al., Id.; Tivol et al., 1995, Immunity 3: 541-7). It has been demonstrated that peripheral T-cell tolerance in vivo requires CTLA-4 engagement (Shrikant et al., 1999, Immunity 11: 483-93; Ratts et al., 1999, Int Immunol 11: 1889-96; Schwarz et al., 2000, J Immunol 165: 1824-31; Chai et al., 2000, J Immunol 165: 3037-42). T-cell activation requires T-cell receptor engagement of its cognate antigen-MHC complex and CD28 binding to the B7 ligands (i.e. CD80 and CD86) on APCs. Activation of T-cells results in increased expression of the T-cell surface molecule CTLA-4 (Bluestone, 1997, J Immunol 158: 1989-93; Linsley et al., 1994, Immunity 1: 793-801). Use of CTLA-4-Ig which blocks co-stimulation has been widely explored for treating autoimmunity and transplant rejection (Londrigan et al., 2010, Transplantation 90: 951-7; Vergani et al., 2010, Diabetes 59: 2253-64). However, this approach down-modulates all activated T-cells and can cause generalized immune suppression. Therefore, this exploration has not been very successful. Because signaling through CTLA-4 down modulates T-cell responses, engagement of CTLA-4 in a targeted fashion has been explored for treating several experimental autoimmune diseases (Karumuthil-Melethil et al., Id.; Fife et al., 2006, The Journal of clinical investigation 116: 2252-61; Li et al., 2007, J Immunol 179: 5191-203; Vasu et al., 2003, Int Immunol 15: 641-54).
Although various pancreatic β-cell specific antigens have been described in the art, none of them is expressed on the cell surface plasma membranes. GLUT2 is a molecule that is expressed on pancreatic β-cell surfaces predominantly (although not completely) where it forms a part of “the glucose sensor” (Thorens et al., 1994, Biochem Soc Trans 22: 684-7). Encoded by the SLC2A2 gene in mammals, GLUT2 is a 524 amino acid multi-pass transmembrane carrier protein. Id. The longest stretch of extracellular domain is 67 amino acids long (UniProtKB/Swiss-Prot entry P11168). GLUT2 is involved in passive glucose transport while not being directly involved in disease pathogenesis and thus is an available target for specific binding to, inter alia, an antibody.